Both silicon substrate and thin film solar cells can be affected by current leakage across the junction. This leakage is called “shunting,” and it reduces the total amount of current that is available to be provided from the solar cell to the load, thereby reducing the efficiency of the solar cell.
A shunted cell can also be harmful when it is electrically connected in a series with other cells to create a module. Because the shunted cell produces less current than the other cells in the module, the shunted cell can become reverse-biased and dissipate an amount of power that can be almost equivalent to the power that is generated by one of the other cells. This power dissipation often causes localized heating or hot spots that can damage the entire module. Hence, solar cells with a low shunt resistance are generally discarded during fabrication, even when the efficiency remains satisfactory.
Shunting is caused by many different types of defects, and appears to be more prevalent in thin films and polycrystalline silicon substrates than in monocrystalline substrates. Although shunts are formed in a variety of ways, they fall into two broad electrical categories: ohmic shunts and weak diodes. Ohmic shunts are characterized by a low shunt resistance in the current-voltage curve for the cell, typically less than about eight ohms. Current leakage across the junction occurs under both forward and reverse biases. Weak diodes are characterized by a low open circuit voltage in the shunted region of the cell, as compared to the nominal open circuit voltage. Under illumination the weak diode region is forward biased by the surrounding area of the cell, causing current to flow in the wrong direction across the junction. Unlike ohmic shunts, weak diodes only demonstrate current leakage under forward bias.
Often the current leakage through the shunt is localized to relatively small regions on the surface or along the edge of the cell. A localized shunt may be screened by the sheet resistance of the top conductive layer. A shunt is screened when the current flowing across the resistive surface towards the shunt creates a voltage drop equal to the difference between the nominal voltage on the surface and the voltage above the shunt. The screening distance to the shunt is that distance across the top resistive surface that is required to create the measured voltage drop. Beyond the screening distance, current no longer flows towards the shunt. Hence a localized shunt effectively removes the region within the screening distance from the active area of the cell.
At low illumination, such as in the morning or evening hours, the screening distance becomes much greater and the degradation of the cell efficiency due to shunting is more serious. The effects described above are complicated when metal fingers have been formed on the surface of the solar cell, to reduce the series resistance of the cell. For example, if the shunt is electrically connected to a metal finger, then current can flow to the shunt from the entire cell. At a low enough shunt resistance, this type of defect may effectively short the entire cell and render it inoperable.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.